Re: Disk images. Was [Solved] Re: Difficulty with a shell function.

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

peter@easthope.ca wrote:
> > Thomas, I cited https://en.wikipedia.org/wiki/Optical_disk_image .
> > ou read https://en.wikipedia.org/wiki/Disk_image .

debian-user@howorth.org.uk wrote:
> When I go to that URL (as shown in my browser's URL bar) I am shown the
> page accessed from the URL below, prefaced by the text "(Redirected
> from Optical disk image)"

Indeed i see the same (and did not notice when i wrote my previous
reply).

It's astounding that wikipedia shows different behavior with the same
URL. If i click on the redirection note i get to
  https://en.wikipedia.org/w/index.php?title=Optical_disk_image&redirect=no
which says not much more than
  "Disk Image
   This page was last edited on 25 July 2008, at 18:54 (UTC)."


peter@easthope.ca quoted previously:
> "... disk image ... written to an optical disk, disk sector by disc sector
> ...".
> Not directly comparable to ext4.

ISO 9660 and UDF usually work with block size 2048. So they are the
natural candidates for being put on optical media which show this
size with their data blocks.

It seems that Linux cannot handle directly ext4 after an image with
default block size has been copied to an optical medium. A workaround
is the use of a loop device on top of the optical drive device.
I tested:

  dd if=/dev/zero bs=1M count=10 of=ext4_dummy.img
  /usr/sbin/mkfs.ext4 -t ext4 ext4_dummy.img
  sudo mkdir /mnt/ext
  sudo mount ext4_dummy.img /mnt/ext
  sudo chmod a+rw /mnt/ext
  echo hello >/mnt/ext/hello
  dd if=/dev/urandom bs=1M count=1 of=/mnt/ext/random_bytes
  ls -l /mnt/ext

This shows

  -rw-r--r-- 1 thomas thomas       6 Jan 30 18:43 hello
  drwx------ 2 root   root     12288 Jan 30 18:39 lost+found
  -rw-r--r-- 1 thomas thomas 1048576 Jan 30 18:48 random_bytes

Now i put it on a DVD+RW:

  sudo umount /mnt/ext
  xorriso -as cdrecord -v dev=/dev/sr0 ext4_dummy.img

After ejecting and loading the medium to kick Linux into examining its
new content, i do:

  sudo mount /dev/sr0 /mnt/ext

which fails with

  mount: /mnt/ext: wrong fs type, bad option, bad superblock on
  /dev/sr0, missing codepage or helper program, or other error.

dmesg reports:

  EXT4-fs (sr0): bad block size 1024

But

  sudo losetup loop0 /dev/sr0
  sudo mount /dev/loop0 /mnt/ext
  ls -l /mnt/ext

yields

  -rw-r--r-- 1 thomas thomas       6 Jan 30 18:43 hello
  drwx------ 2 root   root     12288 Jan 30 18:39 lost+found
  -rw-r--r-- 1 thomas thomas 1048576 Jan 30 18:48 random_bytes

and in dmesg

  EXT4-fs (loop0): mounted filesystem c0ab3f6b-a441-452c-b97f-707842654ed4
  ro with ordered data mode. Quota mode: none.

Cleanup of all created file objects and DVD content:

  sudo umount /mnt/ext
  sudo losetup -d /dev/loop0
  sudo rmdir /mnt/ext
  rm ext4_dummy.img
  xorriso -outdev /dev/sr0 -blank as_needed -eject all


The ISO 9660 filesystem driver of Linux can deal with device block
sizes different from 2048. So no loop device is necessary to mount an
ISO 9660 on a storage device with block size 512 or 4192.


Have a nice day :)

Thomas